1. Field of the Invention
The present invention relates to patient warming systems to prevent and treat hypothermia, especially for surgical or other hospital patients.
2. Description of the Related Art
It is common for certain medical patients to experience mild to severe hypothermia. Some examples include trauma patients, people with a compromised circulatory system, and patients under anesthesia. There are several causes for this decrease of body temperature. First, anesthesia depresses the body""s metabolic rate, meaning that the body burns calories at a reduced rate, and therefore generates less heat. Second, anesthesia may have the effect of depressing the body""s set-point temperature as regulated by the brain""s thermal control center. Third, in a surgical procedure, the patient may have his or her thoracic or abdominal cavities opened, greatly increasing the amount of surface area exposed to the atmosphere as compared to normal, and thereby generating higher body-heat losses from convection and evaporation. Surgical heat loss is further complicated by the cool temperature of many surgical suites.
Current methods of treating hypothermic patients include heating blankets, which use air or liquid as the heat-transfer medium. If placed beneath the patient, heating blanket transfer thermal energy to the patient by a combination of conduction (by direct skin contact with the patient) and convection (via the local air film between the blanket surface and the patient""s skin). Air-type heating blankets are typically placed over the patient, and supply warmed air under low pressure to the blanket, which in turn xe2x80x9cleaksxe2x80x9d the warmed air out at low velocity and with reasonable uniformity over its surface. In both cases, the thermal energy exchange must take place through the patient""s skin surface, relying principally on the vascular system to transfer the thermal energy to the patient""s xe2x80x9ccorexe2x80x9d via blood flow.
The rate of heat transfer, and therefore the effectiveness, of a heat-blanket warming system is necessarily limited for various reasons. One reason is the body""s natural response of xe2x80x9cshutting downxe2x80x9d blood flow to the extremities in the case of low core temperature. In addition to the problem of blood circulation to the extremities being reduced under conditions of hypothermia there are corollary issues to heating blanket methods, as well. One is the necessarily low power density (i.e., watts per unit area) that can be applied without damaging the skin and without eliciting a xe2x80x9csweat response,xe2x80x9d which would counter the intended heat input by sweat evaporation. Another inherent limitation is the physical remoteness of the applied heat from the critical core location. Warm air directed to the arms and legs, for instance, encounters a long series of barriers between the point of applied thermal energy and the core. Thermal energy must first warm the epidermis by convection, then warm the underlying skin layers by conduction. From the underlying skin layers, the thermal energy must be conducted to muscle tissue, and then eventually to the small vessels of the circulatory system (which, as noted, may be constricted as a response to the body""s hypothermic condition).
Physicians also employ intravenous (IV) fluid warmers as another technique for treating hypothermic patients. These devices serve to heat IV solutions that are infused into the patient during surgical procedures. Because of the limitations of volume that can be infused into a patient during typical procedures, these devices lack the capacity to quickly rewarm patients with depressed body temperature.
Consequently, known patient warming approaches are not completely adequate for all applications due to certain unsolved problems.
Broadly, this invention concerns an indwelling radio frequency (RF) catheter that systemically warms patients by dielectric heating of bodily fluids. Although blood is used as a representative example, other bodily fluids are contemplated, such as cerebral spinal fluid. The blood is heated xe2x80x9cin vivo,xe2x80x9d meaning xe2x80x9cwithin the body.xe2x80x9d The indwelling catheter is deployed in a suitable blood vessel, such as the inferior vena cava. The catheter design includes a distally positioned emitter structure electrically coupled to a remote RF generator, which provides a source of RF power. The emitter structure applies a controlled, high-frequency electric field to blood within the patient, thereby creating heat due to the dielectric loss properties of the blood, due in turn to its water content. Advantageously, the emitter may include a bipolar electrode structure, where both poles are present in the patient""s body cavity. Another option is a unipolar emitter structure, which relies upon electric flow between the emitter and the patient""s body. Heating by the indwelling RF catheter continues for a sufficient time that the patient is systemically warmed as blood courses through the patient""s body under influence of the patient""s own circulatory system and/or artificial circulation-aiding machines. The invention may also include various protective or self-distancing structures to prevent the emitter from contacting the surrounding bodily cavity.
The foregoing features may be implemented in a number of different forms. For example, the invention may be implemented to provide a method of increasing a patient""s core temperature by dielectric warming of bodily fluids. In another embodiment, the invention may be implemented to provide an apparatus such as a catheter and/or patient warming system with suitable structure to perform dielectric warming of blood.
The invention affords its users with a number of distinct advantages. For example, the protective and self-distancing features of the invention address the problem of warming a patient at risk for hypothermia without creating clinically significant localized hot spots capable of damaging tissue or blood by protein denaturation. Moreover, this invention provides a greater level of warming beyond that of current thermal-assist technologies, such as externally applied conductive or convective methods. Still another advantage is that the invention applies its heat load nearly directly at the body""s core, where it is most needed in hypothermia. The invention also provides a number of other advantages and benefits, which should be apparent from the following description of the invention.